Animation import into Unity - mesh deform
The method I use is to select in pose mode all (and only) deforming bones, go to Menu Pose/animation/Bake action, and create a new action, clearing all constraints and parenting, so that the Fbx I'm exporting is a clean rig, with no constraints, associated to the clean mesh. Make a copy of both mesh and rig before any attempt (You'll have to try out some exporting preferences, according to the needs). It works fine.